1. Field of the Invention
The present invention relates to a method of production control and, more particularly, to a method of production control that is preferably used for the manufacture of electronic apparatus such as liquid crystal displays.
2. Description of the Related Art
In a Japanese Patent Application (Japanese Patent Laid-Open No. 2001-273023 (hereinafter referred to as “Article 1”), the applicant has proposed a method of production control for production steps that can give and receive capabilities of each other by sharing apparatus in general. According to the method of production control, a load ratio of each of the production steps that is the ratio of the number of products to be processed at the production step to the product processing capability of the production step is first obtained as the ratio of planned cumulative production at the production step to cumulative capability of the production step throughout an operating period that is the object of a medium-term production plan. Next, an optimum ratio of use of each apparatus at each production step is calculated such that the production steps have an equal load ratio. A processing schedule for each production step is determined such that the ratio of use of each apparatus at each production step substantially equals the optimum ratio of use during the operating period.
Article 1 has proposed a method of production control to be used for a group of production steps belonging to a production line for two ore more product types when there is a condition that the history of an apparatus used for processing one product type at one of the production steps limits processing of one product type at another of the group of production steps to an a certain apparatus (such a condition is hereinafter referred to as “condition that limits use of apparatus”). According to this method of production control, the optimum ratio at which each apparatus is used at each production step and for each product type is calculated such that it has an equal load ratio with respect to cumulative processing during an operating period that is the object of a medium-term production plan, thereby reflecting the optimum ratio of use in a short-term production plan.
In general, the method of production control proposed in Article 1 makes it possible to level loads between production steps that can give and receive capabilities of each other by sharing apparatus throughout an operating period that is the object of a medium-term production plan. However, since actual work in process is not considered in calculating a target for processing, a problem arises in that the target quantity to be processed can not be achieved because of shortage of work in process.
According to the method of production control proposed in Article 1, when products are categorized into series of products based on the apparatus that are used or scheduled to be used at the first production step under a condition limiting use of apparatus, processing can be carried out with an optimum ratio if work in process belonging to each series of products is close to the optimum ratio of use of each of the apparatus. However, when work in process is concentrated in any of the series of products, a problem arises in that the quantity that can be actually processed will be significantly smaller than the target quantity to be processed.
When a production step under a condition that limits use of apparatus is a bottleneck production step, in order to maximize processing at the bottleneck production step, it is desirable to feed products to the bottleneck production step from the preceding production step according to a target quantity to be processed for each series of apparatus. However, no existing method of scheduling has allowed maximization of processing at a bottleneck production step in such a way.
In a production line on which product types that are introduced on a batch basis or discrete basis are mingled with product types that are continuously introduced, a problem arises in that a leveled target quantity to be processed that is calculated based on a load ratio leveled throughout a period under a medium-term production plan is not in conformity with a target for processing of the product types that are introduced on a discrete basis.